Fluorescent light bulb based light fixtures are generally rectangular in shape due to the typical long cylindrical shape of the fluorescent bulbs. With the non-symmetric shape of the light fixtures (longer in one direction), the light exiting the fixture using symmetrically scattering diffusers does not typically produce a uniform light illumination.
Improvements in solid state light sources such as light emitting diodes (LEDs) are continuously increasing their efficacy with the luminous output per electrical watt approaching fluorescent sources. LEDs are essentially point light sources as opposed to the extended light sources of fluorescent bulbs. Thus, the light from an LED has the potential to be controlled more effectively, even when more than one LED is used. Additionally, with the move toward point light sources, the visible speckle contrast on the light fixture can increase. More than one diffusive element in the path within the light fixture can reduce this effect, but this technique has not been applied to light fixtures. In other applications when more than one diffuser is used to decrease speckle contrast, symmetric diffusers are typically used and these would inefficiently spread the light in a light fixture. As a result, more light is absorbed within the fixture due to backscatter and the forward scattered light can scatter inefficiently in undesirable directions. Thus, a method is needed to efficiently scatter light while reducing the speckle contrast.
Typical light fixtures used for home, office, outdoor or other lighted areas generally employ traditional diffusive elements or refractive elements to spread the light from the light source. In many situations, it is desired to spread the light from the light source asymmetrically, directing more light along a hallway or path for example. Traditional diffusive elements are surface relief or symmetrically scattering volumetric materials. Surface relief elements can scatter symmetrically or asymmetrically. For example, holographic surface relief diffusers such as those manufactured by Reflexite Inc. or brushed aluminum can be used to asymmetrically spread the light from a light source. Brushed aluminum surfaces are relatively expensive to manufacture compared to the other diffusing options and are not often used. Surface relief diffusers can scatter light asymmetrically, however, all surface relief diffusers have a significant amount of backscatter that can reduce optical efficiency. Additionally, asymmetric surface relief diffusers are typically costly to produce (sometimes requiring expensive holographic mastering and precision casting production techniques) and difficult to clean due to dirt or particles becoming trapped in the inherent holes or pits in the relief structure.
Symmetrically scattering volumetric materials are typically easy to clean but do not efficiently control the scattering of light. They can be free-standing, laminated or otherwise optically coupled to another element such as a substrate, a glass globe, or waveguide because they do not rely on the refractive index difference between the element and air to re-direct light.
Refractive elements are also often used to control the spread of light from a light fixture. These refractive elements can include prism structures, small or micro-lens structures, dimples, indentations or other curved or angled structures one or more surfaces of a film or substrate. The appearance of these structures can create small bright spots in the fixture that are aesthetically unpleasing. Often, the materials used are brittle, creating fragile elements that can break easily. Also, since the refractive elements inherently have at least one surface that is not planar, they are significantly thicker than volumetrically scattering diffusive elements. This can increase the fixture volume and the cost of the fixture. Also, the refractive elements are not easily cleaned due to their surface structure.
Typically, when symmetric or surface relief diffusion is used to scatter the light and make the spatial luminance more uniform, there is a significant trade-off between backscatter and luminance uniformity. Very high isotropic diffusion in the volume or surface will generate a significant amount of backscatter. A significant portion of this backscatter will be absorbed within the fixture such as due to residual absorption in the “white” regions and the light source that do not perfectly reflect the light. As a result, often the uniformity is reduced at the expense of keeping the illuminance high.